1. Field of the Invention
This invention relates to a mounting substrate on which components to be mounted such as an IC chip are mounted and its production method. More particularly, the present invention relates to a mounting substrate which is simple and economical but has a diversified application range and its production method wherein the components to be mounted onto the substrate are bonded by use of an adhesive and thereby being connected electrically to the substrate circuit through metal nodules formed on conductors of the substrate circuit.
Furthermore, the present invention relates to a printed wiring board equipped with connector portions (connection terminal portions) suitable for a fine pitch and having a connector function, and more particularly to a printed wiring board having a connector function and its connection method wherein the substrate is brought into contact with a counter-part conductor through the same metal nodules as mentioned above formed on the conductor of the connection terminal portions and by fixing it by an adhesive, and can thus make connection with an extremely small connection resistance and excellent insulation between adjacent conductors.
2. Description of the Prior Art
When a circuit substrate for surface mounting of IC package such as a flat package (DIP) or a tape carrier is produced, tin-lead electro-plating has conventionally been employed primarily to the conductor pattern portions of the substrate to connect the outer leads of the package by soldering.
More definitely, a cover lay film or a solder resist ink is coated for insulation to the portions of a circuit substrate having a circuit pattern formed on a polyimide film by etching a laminated copper foil on a polyimide film, for example, except the portions of components to be mounted and the connector portions Next, an about 0.2 .mu.m-thick gold is electro-plated onto the terminal patterns for the connection after a Ni (nickel) layer is electro-plated and then solder plating or rustproofing treatment is given to the pads where electrical components are mounted. After the component mounting portions are coated with a paste like cream solder by screen printing method, the components are mounted by use of a mounter and then soldering is accomplished by reflow heating (230.degree. C..times.10 seconds) with far infrared rays in a tunnel type continuous furnace. After the components are thus mounted, the substrate is washed to remove the flux and there is thus obtained finally a mounting substrate.
On the other hand, methods of directly mounting of a bare IC chip can be broadly classified into a wire bonding method and a wireless bonding method.
The wire bonding method is a technology based on connecting the conductor pattern of the substrate to the electrode pads of the IC chip by bonding wires. Bonding of both ends of bonding wires to the IC chip and the substrate is generally conducted by a thermal press system using conjointly an ultrasonic wave.
The wire bonding method is generally used for mounting a bare IC chip to a glass substrate, a ceramic substrate or a glass-epoxy substrate. However, mounting to a flexible substrate is also made in some cases and in this case, the bare IC chip and other chip components are mounted separately from one another. In other words, the bare IC chip is first connected to the substrate by the wire bonding method and molded with a resin. Then, the other chip components are soldered as described above.
The wireless bonding method directly connects the IC chip to the substrate through bumps or the like without using the wires. It includes a flip chip system using the solder bumps, a tape automated bonding (TAB) system using Au bumps, a conductive paste (Ag-Pd) system using Au-plated copper bumps, a conductive rubber connector system, and the like.
More definitely, in the TAB system, about 15 .mu.m-thick Au bumps are formed at the electrode portion of the bare IC chip and the electrode portions of the bare IC chip and finger portions on the film substrate are connected by thermal press through the Au bumps. A package is completed by molding the IC chip by a resin. To further mount this TAB onto a rigid substrate pattern, the outer leads of TAB and the circuit pattern of the substrate must be soldered to one another.
However, the method of the prior art example described above which mounts the IC package or the like by soldering needs an elongated process step as described above. Since reflow heating must be carried out at 230.degree.-260.degree. C. for a few seconds, a heat-resistant polyimide film or the like must be employed in the case of the flexible substrate and an economical material such as a polyester film cannot be used.
As to high density mounting, there is the limit due to the following factors.
Namely, a position error of about .+-.0.2 mm in coating the cover lay film or the solder resist ink and the smear of about .+-.0.2 mm of the adhesive or ink must be taken into consideration and accordingly, the gap between pads (terminals) must be at least about 0.8 mm. Since the cream solder is coated onto the pad, the pad width must be at least 250 .mu.m. If a flat package whose pad gap is of a 0.5 mm pitch is used, the gap is only 250 .mu.m even when the pad width is 250 .mu.m so that the cream solder might swell out when coated and might cause a solder bridge after solder reflow.
On the other hand, the wire bonding method which directly mounts the bare IC chip involves the following various problems.
(1) Though the method is effective for the rigid substrate such as glass, glass epoxy and ceramic substrates, the method has not yet been established as ordinary technique for the application to flexible printed wiring substrates because there are many limitations to the substrate material, plating conditions, and so forth.
(2) Since connection is made for each bonding wire, a long period of time is necessary for mounting.
(3) The width of the conductor wire is limited to 100 .mu.m or above from the aspects of the size of capillary used for thermal press and working property.
(4) Repair cannot be made easily if the IC chip is defective.
(5) The total thickness becomes great due to looping (curve) of the bonding wire such as the Au wire.
In contrast, the conventional wireless bonding method involves the following problems.
(1) Since the bumps and the beam leads must be provided, the cost of production is high.
(2) Repair cannot be made easily and if the IC chip is defective, the substrate must be discarded as a whole.
In the case of the TAB system, the following problems further add to the above.
(3) The electrode pads of the IC chip can be disposed only on the outer peripheral portions of the chip.
(4) The substrate is limited to a film-like substrate and its thickness and width are limited to 125-100 .mu.m and 35-70 .mu.m, respectively.
(5) The heating temperature at the time of bonding is high, e.g. 400.degree. C. for 2 to 5 seconds.
(6) The overall size becomes great when the outer leads are used.
Next, the printed wiring board will be described. Conventionally, when lead wires are taken out from a rigid printed wiring board such as a glass epoxy, paper phenol, transparent electrode glass substrate, ceramic circuit board or metal circuit plate by a flexible printed wiring board, a connection method which connects by soldering or a film-like or sheet-like anisotropic conductor film has most generally been employed.
The anisotropic conductor film is prepared by dispersing conductive particles such as metal powder, plating powder or carbon particles in a hot melt type or heat curing type adhesive and is used after shaped in a film or sheet form or as a liquid. As shown in FIG. 19, for example, when the anisotropic conductor film (or sheet) described above is sandwiched and thermally pressed between a glass substrate 30 having a circuit pattern of a conductor 31 such as ITO (Indium Tin Oxide) film and a polyimide film 26 having formed thereon a circuit pattern of a conductor 2 such as copper, the conductive particles 36 adhere closely because the gap between conductors 31 and 2 becomes small and the conductors 31 and 2 become conductive. Conduction is provided in the direction of thickness while insulation is provided in the planar direction.
However, both the method which uses soldering and the method which uses the anisotropic conductor film should be used when the pitch length between conductors is great, e.g. at least 0.2 mm. If the pitch is smaller than this value, short-circuit will occur between the adjacent conductors and even if it does not occur, insulation between the conductors becomes extremely inferior.
The anisotropic conductor film will be now discussed. As shown in FIGS. 20(a) and (b), insulation drops because the conductive particles exist even between the adjacent circuit patterns (gap portions) and as shown in (a), there is the variance of particle diameters between the conductive particles. Furthermore, since conduction is established by the mutual contact of the conductive particles as shown in (b), contact resistance is great whereas reliability is low. Though the hot melt type adhesive is generally used as the adhesive for forming the anisotropic conductor film, the cost of the conductive particles is by many times higher than the cost of the adhesive alone and the cost of the anisotropic conductor film is therefore high. Still other problems are that the kind of the adhesive used is limited by compatibility with the conductive particles and since the conductive particles are mixed, the bonding power drops than when the adhesive alone is used.
In view of the drawbacks of the prior art described above, it is an object of the present invention to provide a mounting substrate and a production method thereof wherein mounting components inclusive of a bare IC chip are mounted easily and in a high mounting density by a simple process and by use of an economical material.
It is another object of the present invention to provide a printed wiring board having a connector function which has high insulation between adjacent conductors, has an extremely low contact resistance and moreover, can be used for an independent circuit pattern from which leads cannot be extended, and a connection method of such a printed wiring board.